The present invention relates to the field of mass storage devices. More particularly, this invention relates to an improved apparatus and method for writing servo information to the disc of a high density disc drive.
One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are a disc drive housing, a disc that is rotated, an actuator assembly that moves a transducer to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
To read and write data to the disc drive, the actuator assembly includes one or more arms that support the transducer over the disc surface. The actuator assembly is selectively positioned by a voice coil motor which pivots the actuator assembly about a pivot shaft secured to the drive housing. The disc is coupled to a motorized spindle which is also secured to the housing. During operation, the spindle provides rotational power to the disc. By controlling the voice coil motor, the actuator arms (and thus the transducers) can be positioned over any radial location along the rotating disc surface.
The transducer is typically placed on a small ceramic block, also referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (xe2x80x9cABSxe2x80x9d) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equalize so the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on portions of the storage disc referred to as tracks. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto the track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write to or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is often divided between several different tracks. While most storage discs utilize a multiplicity of concentric circular tracks, other discs have a continuous spiral forming a single track on one or both sides of the disc.
During manufacture, servo information is encoded on the disc and subsequently used to accurately locate the transducer. The written servo information is used subsequently to locate the actuator assembly/transducer head at the required position on the disc surface and hold it very accurately in position during a read or write operation. The servo information is written or encoded onto the disc with a machine commonly referred to as a servo track writer (hereinafter STW). At the time the servo information is written, the disc drive is typically at the xe2x80x9chead disk assemblyxe2x80x9d (hereinafter HDA) stage. The HDA includes most of the mechanical drive components but does not typically include all the drive electronics. During the track writing process, the STW precisely locates the transducer heads relative to the disc surface and writes the servo information thereon. Accurate location of the transducer heads is necessary to ensure that the track definition remains concentric.
Servo track writing includes writing a first servo track and then repositioning the transducer to a second position. At the second position, an adjacent servo track is written. The process of moving the transducer from the first position to the second position is referred to as stepping. This stepping process is repeated over and over until the entire disc surface is provided with a number of servo tracks that can be used to located the tracks of data on a disc. Currently, disc drives have discs with up to 2000 tracks in an inch written thereon. A constant desire or industry goal is to increase the storage capacity of disc drives. One way to increase capacity is to increase the density of tracks on the disc. Currently, the number of tracks per inch (TPI) is growing at a rate of 60% per year. It is contemplated that this trend will continue and, as a result, higher track densities will be the norm in the future.
The most common method of stepping during the servo writing process is to move the actuator assembly through a constant angle between the first position and the second position adjacent the first position. The actuator used in the servo write process pivots. Each time the actuator is moved or stepped it is moved through a fixed angle. This method is the quickest way of stepping or repositioning the transducing head used to servo write the disc. Quickness is a factor since the time required to servo write a disc drive is lengthy. In many instances, servo writing a head disc assembly is considered one of the bottlenecks during manufacture. With the increased number of tracks on a disc and the constant trend to increase the number of tracks per inch, the length of time for servo writing discs will also increase. Another pressure is to cut down production time for disc drives.
One problem associated with constant angular stepping is that the track width is wider at the inner diameter of the disc than the track width at the outer diameter of the disc. As a result, the off track performance is much better at the inner diameter of the disc than at the outer diameter of the disc. Off track performance is how well the disc drive reads data when the read head is positioned at a distance from the center of the track. In other words, when reading a track at the inner diameter the read head may be positioned farther off the track than at a track at the outer diameter with the ability to read the data without error or with soft errors.
There is one way to solve the problem of difference in off track performance. The solution has other problems. Rather than step the actuator assembly through a constant angle, the actuator assembly can be stepped so as to produce tracks having the same width (constant track pitch). This process takes more time than the constant angle stepping process since the servo track writer needs to change the step angle for every track written in order to keep the track width (track pitch) constant. This requires the servo track writer to recalibrate the step angle for each track. This approach is impractical for manufacturing purposes since this method adds time to the potential bottleneck of servowriting the disc drive.
Accordingly, what is needed is an apparatus and method for use with a servo track writer (STW) that will allow the servo patterns to be written in tracks so that the off track performance is balanced between the inner diameter and the outer diameter. What is also needed is a disc drive in which the off track performance or capability is more uniform from the tracks at the inner diameter of the disc drive to the outer diameter of the disc drive. What is also needed is a method and apparatus which uses the current servo writing equipment.
The present invention is directed to reducing the problem of having better off track performance at tracks on the inner diameter than at tracks at the outer diameter. The present invention is directed toward evening out the off track performance and the off track capability of the read head at the various tracks across the disc. The present invention includes a method for writing servo patterns on at least one disc in a disc drive that has a first set of tracks and a second set of tracks on a major surface of the disc includes stepping between the tracks of the first set of tracks at a first constant angle, and stepping between the tracks of the second set of tracks at a second constant angle. Stepping between tracks in the first group or the second group includes selecting two adjacent tracks from either the first group of tracks or the second group of tracks, and determining the angle through which an actuator assembly must be rotated between the two adjacent tracks selected to produce a selected track pitch. The result is a series of track sets in which the value of the track pitch varies across the group. The track pitch value for each group is then reset for the next group by recalibrating another constant angle.
The method results in a disc for a disc drive with a first set of tracks, and a second set of tracks in which the track pitch varies across the set of tracks. The track pitch varies between a first value and a second value in the first group. The track pitch varies between a third value and a fourth value in the second group. The first value and the third value are approximately equal as are the second and fourth value of track pitch. The end result is that the track pitch stays more uniform across the surface of the disc.
Advantageously, the method and apparatus of the present invention allows servo patterns to be written on the various tracks of the disc so that the off track performance is balanced between tracks at or near the inner diameter of the disc and tracks at or near the outer diameter of the disc. Using the method and apparatus of the present invention, the off track performance and off track capability is more uniform between the tracks at the inner diameter of the disc drive to the tracks at the outer diameter of the disc drive.